As described, for example, in Patent Literature 1, in a hydraulic shock absorber, a piston chamber in a cylinder housing is filled with oil such as mineral oil, a piston for braking is stored in the piston chamber while an oil circulation gap is held on an outer periphery of the piston, and a rod connected to the piston slidably extends outside through a rod-side end wall (sleeve) of the cylinder housing. When a moving object collides with a distal end of the rod to move the piston, kinetic energy of the moving object is absorbed by flow resistance of oil flowing in the circulation gap.
The rod is normally located at an initial position at a forward end by biasing force of a return spring, and repeats an operation of moving backward to absorb shock when the moving object collides with the rod and then returning to the initial position again because of the biasing force of the return spring. At this time, the oil filled in the piston chamber is attached to a surface of the rod to form an oil film. Hence, when the rod repeats forward and backward movements, the oil in the piston chamber flows outside little by little via the rod. As a result, the oil amount in the piston chamber gradually decreases. Thus, in general, the shock absorbing ability of the shock absorber declines, and the shock absorber comes to the end of its life.
To prevent such decline in the shock absorbing ability due to the decrease in oil amount, the outflow amount is reduced by scraping off the oil film attached to the outer periphery of the rod with a scraper provided between an outer periphery of the rod and an inner periphery of the rod-side end wall and returning the oil film into the piston chamber.
However, on the other hand, the oil film functions as a lubricant oil for a slide between a rod packing provided in the rod-side end wall and the rod. Hence, if the oil film is removed more than necessary, the slide resistance of the rod increases, and the sliding property decreases. Also, the rod packing wears, and sealability decreases.
For this reason, a structure needs to be such that a required amount of oil flows out as lubricant oil, and there is a limit to reduction of the outflow amount of oil. This is the most serious obstruction to prevention of decline in the shock absorbing ability of the shock absorber resulting from the decrease in oil amount.